arXiv subjects

Soonchul Choi

Publications and source records attributed to Soonchul Choi.

10 recordsLinked to original sources

arXiv2026

Nuclear incompressibility and fourth moment of the nuclear density in Skyrme functionals

Recent experimental advances could soon allow the accurate extraction of not only the root-mean-square radius but also the fourth radial moment of the nuclear electric charge density distribution. The fourth radial moment of the nuclear density distribution, $R_4\equiv\sqrt[4]{\left }$, provides a sensitive probe of the nuclear surface thickness, as it is more susceptible to the large-$r$ distributions than the root-mean-square radius ($R_2$). In this work, we examine the utility of $R_4$ for constraining the nuclear equation of state (EoS) at subsaturation densities, specifically for the proton distribution and within the framework of Skyrme energy density functionals. Using a statistical analysis based on predictions from one hundred Skyrme functional models, we demonstrate strong correlations between the energy per particle curvature $K(\rho)$ at $\rho = 0.08 \text{ fm}^{-3}$ and $R_4$ (or the ratio $R_{4/2}=R_4/R_2$) in representative nuclei such as $\text{}^{48}\text{Ca}$ and $\text{}^{208}\text{Pb}$. We establish that $R_{4/2}$, being sensitive to the density tail, serves as an efficient proxy for sub-saturation $K(\rho)$ within the tested Skyrme functional space. Knowledge of $R_{4/2}$ within 0.5\% precision or better, for example in $^{48}$Ca or $^{208}$Pb, could constrain the curvature of the energy per particle of symmetric matter at $0.08$ fm$^{-3}$ within 20 MeV or less.

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arXiv2026

The Sc, Ti, and V Abundance Discrepancy: Testing High-Mass IMF Variation and Massive-Star Rotation

Scandium, titanium, and vanadium can be synthesized primarily in massive stars. Yet many of the current Galactic chemical evolution models under-produce these elements at early epochs. Motivated by evidence that the initial mass function varied in the past on the Galactic disc, we examine how assumptions about massive-star rotation and the initial mass function affect the inferred evolution of Sc, Ti, and V. We compute a grid of one-zone Galactic chemical evolution models that varies the initial rotational velocity of massive stars and the high-mass slope of the initial mass function. We compare the resulting [X/Fe] vs [Fe/H] for X= Sc, Ti, and V tracks and cross-element correlations with Galactic abundance data. We find that adopting rotating massive-star yields with an initial rotational velocity of 300 km/s brings the model trends closer to metal-poor observations, especially for halo stars ([Fe/H] $< -2$), and improves the joint behavior of Sc, Ti, and V. Variations of the high-mass slope of the initial mass function produce a secondary modulation. The remaining tensions, most apparent at solar to super-solar metallicities, motivate future work with a more complete treatment of the enrichment physics and model uncertainties.

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arXiv2024

Deep learning for nuclear masses in deformed relativistic Hartree-Bogoliubov theory in continuum

Most nuclei are deformed, and these deformations play an important role in various nuclear and astrophysical phenomena. Microscopic nuclear mass models have been developed based on covariant density functional theory to explore exotic nuclear properties. Among these, we adopt mass models based on the relativistic continuum Hartree-Bogoliubov theory (RCHB) with spherical symmetry and the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) with axial symmetry to study the effects of deformation on the abundances produced during the rapid neutron-capture process (r-process). Since the DRHBc mass table has so far been completed only for even-Z nuclei, we first investigate whether a Deep Neural Network (DNN) can be used to extend the DRHBc mass table by focusing on nuclear binding energies. To incorporate information about odd-odd and odd-even isotopes into the DNN, we also use binding energies from AME2020 as a training set, in addition to those from the DRHBc mass table for even-Z nuclei. After generating an improved mass table through the DNN study, we conduct a sensitivity analysis of r-process abundances to deformation or mass variations using the RCHB$^\star$ and DRHBc$^\star$ mass tables (where $\star$ indicates that the mass table is obtained from the DNN study). For the r-process sensitivity study, we consider magnetohydrodynamic jets and collapsar jets. Our findings indicate that r-process abundances are sensitive to nuclear deformation, particularly within the mass range of $A=80-120$.

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arXiv2023

{\alpha}-decay half-lives for even-even isotopes of W to U

We investigate {\alpha}-decay half-lives for 74 {\le} Z {\le} 92 even-even nuclei within the semiclassical WKB approximation in deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc). The {\alpha}-particle preformation factors are estimated from cluster-formation model using both empirical AME2020 binding energies and numerical ones obtained by a deep neural network (DNN) study in which available DRHBc binding energies are used as training set. We find that our estimated {\alpha}-decay half-lives are qualitatively in agree with experimental results. We also compare our results with the empirical formulae, ZZCW and UNIV. Based on these observation, we extend our predictions of {\alpha}-decay half-lives for the isotopes whose experimental data are not available.

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arXiv2023

$\Lambda\Lambda$ Interaction in a Nuclear Density Functional Theory and Hyperon Puzzle of the Neutron Star

A Skyrme-type effective potential is determined to describe the interaction between $\Lambda$ hyperons in nuclear medium. Experimental data of the binding energies of the double-$\Lambda$ ($\Lambda\Lambda$) nuclei with mass numbers $A=10$--$13$ are used to fit the parameters of the $\Lambda\Lambda$ interaction. As a result of the fitting, we obtain eight different sets of the $\Lambda\Lambda$ interaction parameters, which reproduces the input data within 5\% deviation from the experimental data on average. The eight $\Lambda\Lambda$ interactions are plugged in the calculation of the heavier $\Lambda\Lambda$ nuclei and the neutron star equation of state to explore the issue of hyperon puzzle. We found that the $\Lambda\Lambda$ interaction, specifically, p-wave interaction makes the equation of state stiff enough that the maximum mass of the neutron star can be as large as, or above $2\;M_\odot$.

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arXiv2023

Effects of neutron-rich nuclei masses on symmetry energy

We explore the impact of neutron-rich nuclei masses on the symmetry energy properties using the mass table evaluated by the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) model. First, using the semi-empirical mass formula with the DRHBc mass table, we investigate the symmetry energy at saturation density $\rho_0$, denoted as $S_0$, and the ratio of surface to volume contributions to the symmetry energy, $\kappa$. As a result, we obtain $S_0=27.85\,{\rm MeV}$ ($\kappa=1.38$) for $a_{\rm sym}(A) =S_0 (1 - \kappa A^{-1/3})$ (Type I) and $S_0=32.66\,{\rm MeV}$ ($\kappa=3.15$) for $a_{\rm sym}(A) = S_0 (1 + \kappa A^{-1/3} )^{-1}$ (Type II), which are lower than those obtained using the AME2020 mass table, $S_0=28.54\,{\rm MeV}$ ($\kappa=1.29$) for Type I and $S_0=33.81\,{\rm MeV}$ ($\kappa=3.04$) for Type II. Second, we further investigate the effect of these changes in $a_{\rm sym}(A)$ on the density-dependent symmetry energy by employing the empirical model of $S(\rho) = C_k(\rho/\rho_0)^{2/3} + C_1(\rho/\rho_0) + C_2(\rho/\rho_0)^{\gamma}$ and universal relation of $a_{\rm sym}(A=208) = S(\rho=0.1\,{\rm fm}^{-3})$. Compared to the experimental constraints, we find that $S_0$ and slope parameter $L$, determined by the DRHBc mass table with Type II, are more suitable to explain the constraints by heavy ion collisions and isobaric analog states than AME2020. We also discuss the neutron skin thickness derived from the $L$, comparing it with experimental measurements.

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arXiv2023

Odd-even shape staggering and kink structure of charge radii of Hg isotopes by the deformed relativistic Hartree-Bogoliubov theory in continuum

We examined the shape staggering of relative charge radii in $^{180 - 186}$Hg isotopes, which was first measured in 1977 and recently confirmed using advanced spectroscopy techniques. To understand the nuclear structure underlying this phenomenon, we employed the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc). Our analysis revealed that the shape staggering can be attributed to nuclear shape transition in the Hg isotopes. Specifically, we demonstrated that prolate shapes of $^{181,183,185}$Hg lead to an increase in the charge radii compared to oblate shapes of $^{180,182,184,186}$Hg isotopes. We explained the nuclear shape staggering in terms of the evolution of occupation probability (OP) of $\nu 1 i_{13/2}$, $\nu 1 h_{9/2}$, $\pi 1 h_{9/2}$, and $\pi 3 s_{1/2}$ states. Additionally, we clarified the kink structure of the charge radii in the Hg isotopes near $N = 126$ magic shell does not come from the change of the OP of $\pi 1 h_{9/2}$ state, but mainly by the increase of the OPs of $\nu 1 i_{11/2}$ and $\nu 2 g_{9/2}$ states.

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arXiv2021

Effects of Many-body Interactions in Hypernuclei with Korea-IBS-Daegu-SKKU Functionals

We investigate the properties of $\Lambda$ hyperon in $\Lambda$-hypernuclei using an effective nuclear density functional theory which is based on the low-energy effective field theory. It expands the energy density in the power of Fermi momentum, and consequently has multiple density dependence for the effective many-body interactions. Starting from the effective density functional for nucleons, we determine the parameters for the two- and many-body $\Lambda$-$N$ interactions added to the nucleon energy density functional by fitting to $\Lambda$-hypernuclear data. The experimental data consist of the energy levels of a $\Lambda$ hyperon in the $p$-, and $d$-states as well as $s$-state of $\Lambda$-hypernuclei in the mass range from $_\Lambda^{16}$O to $_\Lambda^{208}$Pb. The results turn out to properly explain the data relevant to hypernuclei owing to the effective many-body interaction apart from a few data in light hypernuclei. This hyperon functional is applied to study the $\Lambda$ hyperon binding energy of the neutron-rich $^{124-136}_\Lambda$Sn isotopes which are under consideration for the measurement at J-PARC. Our results are shown to be insensitive to the density dependence of symmetry energy. We also examine the nuclear matter including $\Lambda$ hyperon. We note that the hyperon threshold density depends on the nuclear matter properties.

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arXiv2020

Constraints on Nuclear Saturation Properties from Terrestrial Experiments and Astrophysical Observations of Neutron Stars

Taking into account the terrestrial experiments and the recent astrophysical observations of neutron stars and gravitational-wave signals, we impose restrictions on the equation of state (EoS) for isospin-asymmetric nuclear matter. Using the relativistic mean-field model with SU(3) flavor symmetry, we investigate the impacts of effective nucleon mass, nuclear incompressibility, and slope parameter of nuclear symmetry energy on the nuclear and neutron-star properties. It is found that the astrophysical information of massive neutron stars and tidal deformabilities as well as the nuclear experimental data plays an important role to restrict the EoS for neutron stars. Especially, the softness of the nuclear EoS due to the existence of hyperons in the core gives stringent constraints on those physical quantities. Furthermore, it is possible to put limits on the curvature parameter of nuclear symmetry energy by means of the nuclear and astrophysical calculations.

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arXiv2017

Effects of pairing correlations on the neutron skin thickness and the symmetry energy

We investigated effects of pairing correlations on the neutron skin thickness and the symmetry energy of finite nuclei. In this calculation we used Hartree-Fock-Bogoliubov (HFB) method with Skyrme forces and effective pairing interactions. The results have been compared with available experimental data, Hartree-Fock (HF) results as well as the predictions by droplet model (DM). Finally, our discussion was extended to study of the pairing interaction in nuclear matter. Roles of isospin T = 0 pairing in the nuclear matter were also discussed.

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